TEMPERATURE MEASUREMENTTEMPERATURE MEASUREMENT.pptx

TEMPERATURE MEASUREMENT

1. TEMPERATURE MEASUREMENT

2. Definition of temperature Temperature is defined as the degree of hotness or cold ness of body is called temperature. Or Temperature is an indication of intensity of molecular activity. Or A condition of a body by virtue of which heat is transferred to or from other bodies.

3. TEMPERATURE SCALES IN SI SYSTEM Temperature scales are based upon some recognized fixed points. At least two fixed points are required which are constant in temperature and can be easily reproduced as: Centigrade scale Fahrenheat scale Absolute scale Kelvin scale Rankine scale International practical temperature scale

4. 1) Centigrade scale: The freezing point and boiling point of water are used as fixed point. Freezing point or ice point 0 C Boiling point or Steam point 100 C 2) Fahrenheat scale: Freezing point or ice point 32 F Boiling point or Steam point 212 F 3) Kelvin scale: In SI system absolute scale kelvin scale. T(K) = T( C ) + 273.15

5. 6. International practical temperature scale:- This scale has been established and adopted provide an experimental basis for the calibration of specific thermometers to indicate temperatures as close as possible to the Kelvin thermodynamic scale. The International temperature scale covers the range from the boiling point of oxygen to the highest temperatures of incandescent bodies and names. The main features of this scale, adopted in 1948 at the Ninth General Conference on Weights and Measures are: (1) Temperatures are to be designated as of and denoted by the symbol t. The name Celsius wa s officially adopted to replace the name Centigrade. (2) The scale is based upon a number of fixed and reproducible equilibrium temperatures to which numerical values are assigned.

6. The standard instruments used for measurement of temperature 0-630 C Platinum Resistance Thermometer is used. 630-1064 C 10 % Rhodium/ Platinum Thermocouple is used. Above 1064 C By using plank law of radiation.

7. Characteristics for temperature measurement: Increase or decreases in size. Increase in pressure if enclosed. Change in colour . Change of state. Change of electrical resistance. Generation of emf . Change in degree of surface radiation.

8. CLASSIFICATION OF TEMPERATURE MEASURING INSTRUMENTS Based on the changes produced in the physical properties of a material due to the change in temperature, the temperature measuring instruments are classified as follows Expansion Thermometers (Non-Electrical) Electrical Thermometers Radiation pyrometers

9. Expansion thermometers (Non-Electrical): i ) Liquid in glass Thermometer ii) Bimetallic Thermometers iii) Pressure Thermometers a) Liquid filled b) Gas filled c) Vapour filled 2. Electrical thermometers i ) Resistance thermometers a) Resistance Temperature detector (RTD) b) Thermistors or Semiconductor sensors ii) Thermo couples 3. Radiation Pyrometers i ) Total Radiation Pyrometers ii) Optical Pyrometers

10. 1. Expansion thermometers (Non-Electrical) i ) Liquid in glass Thermometer LIQUID

11. Works on the principle of change in physical dimensions of a liquid. Commonly used liquids are Mercury and Alcohol. Properties of liquids used in Liquid in glass thermometer: They should have large coefficient of expansion. The liquid should not adhere to the capillary walls. The liquid should have linear relationship between dimensional change and temperature. The liquid should be able to operate with in a reasonable temperature range without any phase change. The liquid should be a clearly visible. Cost of liquid should be low.

12. Salient features or Advantages: Portable in size. No need of auxiliary power. Good response accuracy and sensitivity. Low cost. Fragile construction. Remote indication is not possible. Range is limited to 650 C. High time lag.

13. ii) Bimetallic Thermometers: These thermometers are use the two principles All metals are change in dimensions that is expand or contract when there is change in temperature. The difference in thermal expansion rates used to produce deflections which is proportional to temperature changes.

14. Fig: Bimetallic strip

15. Bimetallic thermometer material properties: Coefficient of expansion. Modulus of elasticity. Elastic limit after cold rolling. Electrical conductivity. Ductility. Metallurgical ability. Generally used Bimetallic materials Brass, Nickel-Iron alloys with Chromium and Manganese used for High expansion. Invar(Alloy of Nickel and Iron) used for Low expansion.

16. Different types of bimetallic sensors

17. There are two types of bimetallic thermometers Helix Bimetallic Thermometer Spiral Bimetallic Thermometer

18. 1) Helix Bimetallic Thermometer

19. 1) Spiral Bimetallic Thermometer

20. Advantages: They are simple, robust and inexpensive. There accuracy is between + 2 to + 5 % of the scale. They can withstand 50% over range in temperatures. They can be used wherever a mercury in glass thermometer is used. Dis advantages: Not recommended for temperatures above 400 C. Introduce Errors when regularly used. Applications: Bimetallic strips are used in control device. Spiral strip used in Air conditioning thermostats. Helix strips used for process application such as Refineries, Oil burners, tyre Vulcanisers.

21. iii) Pressure thermometers Types of liquids used for Pressure thermometer Liquid filled: In this thermometer used fluids are Mercury, Ethyl Alcohol, Ether, Tolune . Gas filled: In this thermometer used fluids are Nitrogen, Helium. Vapour filled: In this thermometer used fluids are Argon, Di-Ethyl Ether, Ethyl Alcohol, Methyl Chloride, Water.

22. i ) Liquid filled Thermometer or (Constant Volume Gas Thermometer)

23. ii) Gas filled Pressure Thermometer

24. iii) Vapour Pressure Thermometer

25. Pressure Thermometer

26. Advantages: cost is less. Speed of response is high. Sensitivity is high. These devices are rugged. Controlling of devices and direct driving of recording is possible due to the large force output. Limitations: Long transmitting capillary tubes are used. The filling fluid and tube are temperature sensitive which may introduce output errors. Sensing bulb is placed at considerable height. The filling fluid might get decomposed introducing calibration drift.

27. 2. Electrical Thermometers In this thermometers are works on the principle of change in some electrical properties when they are subjected to temperature change. i ) Resistance Thermometer:

28. The resistance of a conductor changes when its temperature is changed. This change occurs mainly due to Dimensional change Change in materials current opposite properties The resistance thermometers are classified in to two types Resistance Temperature Detector(RTD) Thermistors or Thermal resistors or Semi conductor resistance sensors.

29. i ) Resistance Temperature Detector(RTD) Electric conductor is subjected to temperature change, the resistance of the conductor changes. This change in resistance of the electric conductor becomes a measure of the change in temperature when calibrated.

30. Different resistance element materials are Platinum, Nickel, Copper, Iron alloy. Properties of sensing materials: Fabrication in convenient sizes. High coefficient of resistance. Should be corrosive resistance. Should not undergo Phase change at high temperatures in the range. Low cost.

31. Fig: Resistance temperature detector(RTD) Applications: Temperature measurement Used in continuous monitoring situations.

32. Advantages: Simple in construction. Accurate measurement. Average temperature measurement is possible. Very easy to install and replace the thermometer. Measuring circuit can be checked easily. Easy reproducibility. Disadvantages: Slow in response. Current leakage might take place between the thermometer and ground. Lead resistance compensation becomes essential. Thermoelectric emf may generated due to a junction of two dissimilar metals.

33. ii) Thermistors or Thermal resistors or Semi conductor resistance sensors

34. Types of thermistors Eye Type

35. Thermistor materials: Thermistors are made of metallic oxides of copper, Iron, Nickel. These metallic oxides are mixed with Binders, pressed to required shapes and then they are sintered. Advantages: Cost is less. Accuracy is High. Measure the High temperatures. They posses the ability to withstand Mechanical and Electrical stresses. Manufactured very small sizes. Simple electric circuit is used.

36. Dis Advantages: Non linear scale. the resistance of the thermistors increases when time lapses. Current passes through the thermistors, it gets heated. Applications: Good Sensitivity. Used for temperature compensation in electronic equipment. Used in time delay circuits. used to measure pressure and flow of liquids.

37. THERMOCOUPLES The most commonly used electrical temperature measuring instrument is thermocouple. There are three effects used in thermocouples Seebeck effect Peltier effect Thomson effect

38. Seebeck effect: Peltier effect & Thomson effect Peltier Thomson Thomson

39. Thermocouple arrangement for measuring temperature

40. Laws of thermocouples There are three laws of thermocouples Law of thermoelectricity or Successive or Intermediate temperatures Law of Intermediate metals Law of homogeneous circuit: The application of heat to a single homogeneous metal is in itself not capable of producing or sustaining an electric current.

41. 1) Law of thermoelectricity or Successive or Intermediate temperatures

42. 2) Law of Intermediate metals

43. Thermocouple materials: The common materials used for thermocouple are Copper, Iron, Platinum, Rhodium, Iridium, Constantan, Chromel, Alumel, Boron and Graphite.

44. Thermocouple construction Bare and Insulated Thermocouples Twisted and Bare Resistance Welded Bare Butt Welded and Bare Thermocouple with single hole round insulation Thermocouple with Double hole round insulation Thermocouple with Double hole tube insulation

45. Thermopile or Thermocouples connected in series

46. Thermocouples connected in parallel

47. 3. RADIATION PYROMETERS Pyrometry means temperature measurement using various forms of thermal radiation measurements. Measures the high temperatures. Non contact measuring instruments are used. Pyrommetry deals with the measurement of temperatures are above 500 C. The instrument used to measure the such high temperatures are called pyrometers. There are two types of radiation pyrometers Total radiation pyrometer Optical radiation pyrometers

48. Total radiation pyrometer

49. Focusing for thermal radiation Multi volt meter Parabolic reflector Thermocouple Thermocouple Total radiation Multi volt meter

50. Advantages: They can measure the temperature of radiating object without any physical contact with the radiating object. High speed of response. Used to measure temperatures of stationary and moving objects. They have a high accuracy of + 2% of the scale range.

51. Disadvantages: The presence of dust, smoke, and gases between the radiating object introduce the errors. The presence of hot gases, flames etc.. Between the radiating object and the concave mirror will make the pyrometer to read high. Low sensitivity in the lower temperatures. Cannot be used for temperature lower than the 500 C. Cooling is required to protect the instrument for over heating. Intensity of radiation decreases after a certain distance.

52. Applications: Measure the high temperatures ranging from 1200 C to 1500 C.But general they are used in the range of 700 C to 2000 C.

53. Optical pyrometer

54. Filament (Dark) cooler than temperature source Filament (Bright) Hotter than temperature source Filament (Disappears) Equal brightness of filament temperature source

55. Advantages: Physical contact of the instrument is not required. Accuracy is high 5 C. Provided a proper sized image of the temperature source. Easy to Operate. Dis advantages: Only to measure the more than 700 C. It cannot be used for continuous monitoring and controlling purposes. Applications: Used to measure temperatures of molten metals and heated materials. Used to measure temperature of furnaces.